Early Hydration of Mixtures of MgO and Hydromagnesite

Abstract

This work investigates the hydration of mixtures of magnesium oxide (MgO) and hydromagnesite (HY), with a particular focus on the formation of the recently described hydrous carbonate-containing brucite (HCB) phase. Whereas MgO normally hydrates to brucite in aqueous media, the addition of a hydrated magnesium carbonate such as HY promotes the formation of an amorphous brucite-like structure that can incorporate both water and carbon into its interlayers and that exhibits characteristic stacking faults. The influence of the MgO to HY ratio on HCB formation was examined by heat-flow calorimetry, ¹H-TD-NMR, in situ XRD, and pore-solution analysis. The results show that HY strongly accelerates hydration: in the mixtures examined (MgO : HY = 70 : 30 and MgO : HY = 90 : 10), the main reaction, leading to HCB formation, takes place within the first five hours. The amount of HCB formed remained essentially independent of HY content. Even at the higher HY content (70/30), no larger extent of reaction was observed. Only small amounts of dissolved carbon were detected in the pore solution although the theoretical amount available from partial HY dissolution was much higher. This discrepancy can only be explained by structural incorporation of carbon into the HCB phase. In addition, ¹H-TD-NMR identified three water reservoirs in the MgO/HY system: free water, gel-like water in nanopores of HCB agglomerates, and interlayer water structurally bound in the HCB phase.